For example, Patent Literature 1 describes a hydraulic drive system for a construction machine comprising a pump device which has two delivery ports whose delivery flow rates are controlled by a single pump regulator, and a load sensing system which controls delivery pressures of the pump device to be higher than the maximum load pressure of actuators. In the Patent Literature 1, a hydraulic pump of the split flow type is used as the pump device having two delivery ports. The split flow type hydraulic pump, including only one pump regulator and only one swash plate (displacement control mechanism), controls the delivery flow rates of the two delivery ports by adjusting the tilting angle of the single swash plate (displacement) with the single pump regulator, thereby implementing a pump function of two pumps with a compact structure.